The present invention relates to fluid coupling devices, and more particularly, to fluid coupling devices of the type including modulating valving.
Although the present invention may be used advantageously in fluid coupling devices having many different configurations and applications, it is especially advantageous in a coupling device of the type used to drive a radiator cooling fan of an internal combustion engine, and will be described in connection therewith.
Fluid coupling devices of the viscous shear type have been popular for many years for driving engine cooling fans, primarily because their use results in a substantial saving of engine horsepower, when compared to a conventional shaft driven fan. The horsepower savings results from the fact that the viscous coupling operates in the engaged, full speed condition only when cooling is needed, and operates in a disengaged, relatively lower speed condition when little or no cooling is required.
In an effort to effect even greater savings of engine horsepower, those working in the art developed "modulating valving" for fluid couplings of the type to which the invention relates. See for example U.S. Pat. No. 3,227,254, assigned to the assignee of the present invention. Briefly stated, modulating valving has the effect of moving the fluid inlet opening radially outward as the temperature increases, to progressively increase the volume of fluid in the operating chamber of the device, as the ambient air temperature increases over a predetermined range.
However, in applying modulating valving to various sizes and configurations of fluid coupling devices, it has been found that frequently the use of modulating valving alone is not sufficient to achieve the desired fan speed vs. temperature relationship. Instead, fan speed (fluid coupling output speed) rises more rigidly with increasing temperature than is desirable.